Many devices and instruments for geodetic or industrial measurement comprise tilt detectors for allowing an alignment with or an orientation relative to the direction of gravity.
There are many different kinds of tilt detectors known from prior art. For example, some are working with pendulums that are supported mechanically or electromagnetically, a position of which being detected by electronic, inductive, capacitive or optical means. Microelectronic mechanical systems (MEMS) typically use spring-mass systems that detect a displacement of a small test mass in dependence of the position. Other tilt detectors are based on liquids and use a reflection or refraction of a ray of light on the liquid level, a resistance measurement or capacitive measurements in dependence of the position of the liquid to determine a tilting angle.
As the Earth is neither perfectly spherical, nor of perfectly homogenous composition, nor motionless in space, the direction of the gravity vector of a point on the Earth almost nowhere exactly meets the centre of the Earth. Also, the gravitational acceleration differs from point to point.
At the Earth's surface the average gravitational acceleration accounts for approximately 9.81 m/s2. This is an average value, as the Earth is not formed spherically but more like an ellipsoid of revolution, so that the poles are nearer to the Earth's centre than the equator. Moreover, the Earth rotates, so that the nearer a point of the Earth's surface is to the equator, the lower is the local gravity due to the centrifugal force; at the Earth's poles this effect is missing completely. Summed up, as a result of these two effects, on the Earth's surface gravity is by about 0.5% larger at the poles than at the equator. So, the gravitational acceleration is 9,832 m/s2 at the poles and 9,745 m/s2 at the equator.
Additionally, the inhomogeneous composition of the Earth's crust causes an inhomogeneous distribution of the direction and amount of gravity on the Earth's surface. For example, large masses of especially high density, such as nearby mountains or large ore deposits in the subsurface, can lead to local gravity anomalies. The measure of how far the direction of the local gravity field of a point on the Earth has been shifted by such local gravity anomalies is the “vertical deflection”. This is the local difference between a local gravity vector, i.e. the true plumb line, and a reference vector, i.e. the line that would be perpendicular to the surface of a reference ellipsoid, e.g. chosen to approximate the Earth's sea-level surface.
For geodetic measurement purposes in which a point on the Earth's surface is to be measured with respect to a reference coordinate system, such as the mentioned reference ellipsoid, especially if high precision is required, vertical deflection can cause a serious problem, as tilt detectors of geodetic devices only detect the local plumb direction, i.e. the direction of the local gravity vector, and not the direction of a vector referenced to the reference coordinate system.
Geodetic survey poles, for instance, need to be placed on a point to be measured in such a way that a reflecting part is centred over the point. A user holds the survey pole in a vertical position on a point to be surveyed using a bubble level so that the retro-reflector is directly over the point to be surveyed. Unless the pole is perfectly upright with respect to the ground, the horizontal position of the reflector will be displaced compared to the location on the ground of interest.
Classically, also pitch and roll of the pole are observed through measurements of the local gravity vector which induce an acceleration measurement on the accelerometers.
A geodetic survey pole for use with a geodetic surveying device, such as a total station, and comprising a tilt detector and a position detection device, such as a GNSS antenna, for instance, is disclosed in the documents US 2009/0024325 A1 and U.S. Pat. No. 5,929,807.
Through entering known values for the local gravitational direction and/or acceleration in a device's setup, the vertical deflection effect could be at least partially compensated. Compensation software of the device then could automatically compensate the undesired effect by calculating corrected inclination values based on the entered gravity values or vertical deflection data. However, for this solution the gravity values or the vertical deflection data for the current location would have to be known to the user or at least be available to the user at the time of the measurement. Also, this method would be rather time-consuming and through input errors a consistent source for measuring errors. In a simpler solution, only the (approximate) latitude could be entered, thus however completely neglecting the effects of local anomalies.